A conventionally known planetary rotary/linear motion converter for converting a rotary motion into a linear motion is disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. 10-196757. The planetary rotary/linear motion converter has a multi-thread screw sun gear, screw planetary gears, and a screw ring gear, which cooperatively constitute a planetary gear mechanism. The external thread of the screw sun gear and the internal thread of the screw ring gear are of opposite helical directions. The screw sun gear, the screw planetary gears, and the screw ring gear are set in a certain relationship in terms of the number of threads and the pitch diameter of threads.
Also, a conventionally known planetary differential screw-type rotary/linear motion converter is disclosed in International Publication No. WO2004/094870, which was filed by the present applicant. The planetary differential screw-type rotary/linear motion converter has a multi-thread screw sun gear, screw planetary gears, and a screw ring gear, which cooperatively constitute a planetary gear mechanism. The screw sun gear and the screw planetary gears mesh with each other through threads of opposite helical directions. The screw sun gear and the screw ring gear mesh with each other through threads of the same helical direction. The screw sun gear, the screw planetary gears, and the screw ring gear are set in a certain relationship in terms of the number of threads and the pitch diameter of threads. Through utilization of the principles of the planetary gear mechanism and the principles of differential gears, the planetary differential screw-type rotary/linear motion converter accurately converts a rotary motion into a linear motion.
U.S. Pat. No. 2,683,379 discloses a planetary rotary/linear motion converter similar to the motion converter disclosed in International Publication No. WO2004/094870. The planetary rotary/linear motion converter has a screw sun gear, screw planetary gears, and a screw ring gear, as well as planetary spur gears and a ring spur gear. The screw sun gear and the screw planetary gears mesh with each other through threads of the same helical direction (coincidental helical direction).
The above-mentioned conventional rotary/linear motion converter disclosed in Japanese Patent Application Laid-Open (kokai) No. 10-196757 is prone to unstable operation which is caused by sliding between the screw sun gear and the screw planetary gears or a like cause, and is therefore not feasible for practical use. The above-mentioned rotary/linear motion converter disclosed in International Publication No. WO2004/094870 can accurately convert a rotary motion into a linear motion, but requires a complex structure for ensuring reliable operation. Furthermore, the converter cannot convert a linear motion into a rotary motion. In the above-mentioned rotary/linear motion converter disclosed in U.S. Pat. No. 2,683,379, an advancing amount of the sun shaft associated with the rotation of the ring shaft depends on the number of teeth of the screw sun gear, the number of teeth of each screw planetary gear, and the number of teeth of the screw ring gear. Therefore, the conversion ratio of the amount of a linear motion to the amount of a rotary motion cannot be set freely. Furthermore, the efficiency of conversion is low.